1. Field of the Invention
This invention relates to a velocity control apparatus for controlling the velocity of a servomotor based on a velocity signal feed back from the servomotor.
2. Description of the Related Art
Servomotors for NC machine tools, robots and the like often employ a control method which relies more upon a digital servo system, in which digital processing is executed based on digital commands, rather than upon an analog system.
FIG. 3 is a block diagram illustrating an example in which a servomotor is controlled by such a digital system. A command signal formed by a host system a such as a computer is applied to a digital controller b. The digital controller b includes by a pulse shaper and direction discriminating circuit bl, a deviation counter b2, and a D/A converter b3. A position signal detected by a pulse coder e mounted on a servomotor d is fed back to the digital controller b, so that a deviation signal between the detected position signal and the input command signal may be formed. The deviation signal is converted into an analog signal by the D/A converter b3, and the analog signal is delivered to a velocity control unit c. The velocity control unit c includes by a velocity controller c1 and a current controller c2. The velocity controller c1 obtains a current command signal by comparing a velocity signal fed back from a tachogenerator f with the analog value of the deviation signal, and applies the current command signal to the current controller c2. The current controller c2 receives current feedback from the servomotor and, by comparing this current with the current signal obtained by the velocity controller c1, forms a drive control signal for the servomotor d.
In general, therefore, the velocity control unit c includes a velocity control loop and a current control loop composed of analog circuitry which performs processing to produce the servomotor drive control signal. With the recent popularization of 16-bit microcomputers, however, digital servo systems have been developed in which all processing for such velocity control and current control is performed digitally.
The reason for the latter is that many advantages are obtained in terms of control precision, system cost and the like when the servo control system, inclusive of the velocity control loop and current control loop, is composed entirely of digital circuitry. However, a computer that comprises such a servo system includes software and is required to store parameters for servo system gain adjustment, velocity loop compensation and the like in a memory. The computer is also required to carry out various processing control with regard to the processing of feedback data such as velocity signals and position signals after sampling, at a prescribed period, and with regard to information actually generated by a signal detecting mechanism such as the pulse coder. Accordingly, a problem that arises is a delay in such signal processing even is a 16-bit microprocessor having a high processing capability is used. The reason is that when the above mentioned data are sampled at a predetermined sampling rate so as to accommodate the processing performed by the microprocessor, a lag element results when the data are accepted within the computer as velocity information and thereby causes an inaccuracy in servomotor control.